The use of omnidirectional vision has increased during these past years. It provides a very large field of view. Nevertheless, omnidirectional images contain significant radial distortions and conventional image processing is not adapted to these specific images. This paper presents an edge detector adapted to the image geometry. Fuzzy sets will be used to take into account all imprecisions introduced by the sampling process. The Prewitt filter applied to omnidirectional image will be studied to illustrate this paper.